(1) Field of the Invention
The present invention relates to a water-in-oil emulsion explosive composition, and more particularly relates to a cap-sensible water-in-oil emulsion explosive composition containing a gas-retaining agent consisting of bubble assemblies, each of which assemblies is one particle consisting of a large number of bubbles agglomerated into the particle, and having a very low detonation velocity, a very high safety against methane and coal dust and an excellent sympathetic detonation performance without lowering the strength.
(2) Description of the Prior Art
Various investigations have been recently made with respect to water-in-oil emulsion explosive (hereinafter, abbreviated as W/O explosive). For example, as disclosed in U.S. Pat. No. 3,161,551 and No. 3,447,978, the W/O exposive has an emulsified structure consisting of a continuous phase which consists of a carbonaceous fuel, and a disperse phase which consists of an aqueous solution of inorganic oxidizer salt, such as ammonium nitrate or the like, and is entirely different in the structure from hitherto been known oil-in-water slurry explosive (hereinafter, abbreviated as O/W explosive).
That is, O/W explosive has an oil-in-water structure, wherein an aqueous solution of inorganic oxidizer salt, a sensitizer and the like are dispersed in the form of a gel together with a gelatinizer as described, for example, in Makoto Kimura, "Slurry Explosive, Performance and Use Method", Sankaido (1975). On the contrary, W/O explosive has a water-in-oil microfine structure, wherein microfine droplets consisting of an aqueous solution of inorganic oxidizer salts and having a particle size of 10 .mu.m-0.1 .mu.m are covered with a very thin film of oil consisting of a carbonaceous fuel and a surfactant as described, for example, in Kogyo Kayaku Kyokai-Shi, 43 (No. 5), 285-294 (1982).
W/O emulsion is remarkably different from O/W emulsion in the performance and composition due to the above described difference in the structure. That is, O/W explosive requires to contain a sensitizer, such as aluminum (U.S. Pat. No. 3,121,036), monomethylamine nitrate (U.S. Pat. No. 3,431,155 and No. 3,471,346) or the like, and is relatively low in the detonation velocity. On the contrary, W/O explosive is good in the contact efficiency of the carbonaceous fuel with the inorganic oxidizer salt, and hence the W/O explosive has excellent properties. For examples, the W/O explosive is high in the detonation velocity, has cap-sensitivity in itself without containing sensitizer, is good in after-detonation fume, and can be changed widely in its consistency.
However, in order to maintain cap-sensitivity, propagation property of detonation, sympathetic detonation property and the like in a W/O explosive, that is, in order to ensure the detonation reliability of the explosive, it is necessary to adjust the specific gravity of the explosive by containing bubbles therein.
As the gas-retaining agent, hollow microspheres, each consisting of a single independent bubble, have hitherto been used. For example, U.S. Pat. No. 4,110,134 discloses the use of glass hollow microspheres or Saran resin hollow microspheres, both of which consist of single independent bubbles having a particle size of 10-175 .mu.m; and a U.S. patent application filed July 5, 1984 discloses the use of resin hollow microspheres, each consisting of a single independent microsphere having a small particle size of not larger than 175 .mu.m. All of these prior arts use hollow microspheres, each consisting of a single independent bubble having a small particle size. However, the W/O explosives containing these gas-retaining agents are high in the detonation velocity, and the production of W/O explosives having a high safety against methane or coal dust has been impossible. Moreover, hollow microspheres, each consisting of a single independent bubble, are very expensive, and it has been technically and ecconomically difficult to produce a W/O explosive having a low detonation velocity by using a large amount of the hollow microspheres.
The use of shirasu hollow microspheres obtained by firing volcanic ash and the like as a gas-retaining agent is disclosed in various prior arts (for example, Japanese Patent Laid-open Application No. 84,395/81). As the shirasu hollow microspheres, there are known shirasu hollow microspheres, each consisting of a single independent bubble, or shirasu hollow microspheres consisting of bubble assemblies, each bubble assembly being a secondary particle consisting of a relatively small number of bubbles fused to each other. However, these shirasu hollow microsphres are low in the effect for lowering the detonation velocity of a W/O explosive and were not able to attain a high safety against methane and coal dust in the resulting W/O explosive.
Alternatively, U.S. Pat. No. 4,008,108 discloses a method for producing a W/O explosive containing simple bubbles by adding a foaming agent or gas-generating agent to the raw material mixture during the production of the explosive or by blowing bubbles into the raw material mixture during the production thereof under mechanical stirring in place of the use of these gas-retaining agents. However, the simple bubbles as such can not be contained in the resulting W/O explosive in an amount more than a certain amount, are difficult to be contained in the W/O explosive for a long time, and leak from the explosive with the lapse of time, and hence the explosive loses its cap-sensitivity, deteriorates in a short time, and is not advantageous for practical use.
As described above, the production of a W/O explosive having a low detonation velocity is very difficult as compared with the production of an O/W explosive having a low detonation velocity. However, it is indispensable to produce an explosive having a low detonation velocity in order to produce an explosive having a safety against methane and coal dust.
A most general method for producing a W/O explosive having a low-detonation velocity is to produce a W/O explosive having a low specific gravity. In order to produce an explosive having a low specific gravity, it is necessary to contain a large amount of gas-retaining agent in the explosive. For example, even when a large amount of the above described hollow microspheres are used so as to contain 40% by volume, based on the volume of the resulting W/O explosive, of bubbles in the explosive, a W/O explosive having a detonation velocity of not higher than 3,000 m/sec can not be obtained. Moreover, the use of such large amount of expensive gas-retaining agent is not ecconomical, and results in a W/O explosive having a very low strength and a very poor detonation reliability, and the explosive can not be practically used. Further, there is known a method for lowering greatly a strength of an explosive in order to obtain a high safety against methane and coal dust in the explosive (for example, Japanese Patent Laid-open Application No. 155,091/81). For example, there is known a method, which uses a large amount of an inactive substance of flame coolant, such as sodium chloride, water or the like. However, in this method, a W/O explosive having a detonation velocity of not higher than 3,000 m/sec can not be obtained, and due to the presence of a large amount of such inactive substance, the resulting W/O explosive has a broken fine structure, deteriorates rapidly with the lapse of time and has no cap-sensitivity.
As an effective method for securing a high safety against methane and coal dust of a W/O explosive without deteriorating its strength, there is known a method which uses hollow microspheres having a relatively large particle size as a gas-retaining agent. However, hollow microspheres, each consisting of a single independent bubble, or bubble assemblies, each assembly being one particle consisting of less than 10 relatively small bubbles agglomerated into the particle, become lower noticeably in their strength corresponding to the increase of their particle size. For example, silica hollow microspheres having an average particle size of 600 .mu.m are easily broken during the production of explosive, and damages the production installation for the explosive. Moreover, fragments of the silica hollow microspheres break the microfine structure of W/O explosive, and the resulting W/O explosive is deteriorated in its performance with the lapse of time. In addition, a W/O explosive containing such hollow microspheres is easily broken due to the pressure caused by the explosion in an adjacent bore hole at the blasting, and is apt to cause dead pressing. In order to obviate this drawback, it has been proposed to use strong hollow microspheres having a large wall thickness and a relatively large particle size. However, such glass hollow microspheres are difficult in obtaining them in the market, are expensive, and further have a large specific gravity and must be contained in a large amount in a W/O explosive, and the resulting W/O explosive is poor in the initiation performance and has not a satisfactorily low detonation velocity.
As described above, a W/O explosive has a high detonation velocity due to its microfine structure, and it is difficult to produce having a low detonation velocity by containing in it conventional hollow microspheres, each microsphere consists of a single independent bubble, and it is impossible to produce a W/O explosive surely having a high safety against methane and coal dust.
When ordinary explosive is used in a place, wherein combustible gases, such as methane and the like, or combustible dusts, such as coal dust and the like, are present, there is a risk of gas explosion or dust explosion. Such operation site, for example, coal mine or like is in duty bound to use an explosive having a safety higher than a given safety standard. In order to produce an explosive having a high safety against methane, coal dust and the like, it is indispensable to decrease the strength of explosive and further to decrease the detonation velocity. Particularly, in a W/O explosive having a relatively high detonation velocity, in order to obtain the same safety as that of O/W explosive, the strength of the W/O explosive must be extremely lowered. However, such W/O explosive is poor in the detonation reliability sympathetic detonability and storage stability, and can not be practically used. Moreover, the use of an explosive having a low strength is poor in the mining effect and increases the number of blasting times, resulting in an increased danger.
The inventors have variously studied in order to produce a cap-sensitive W/O explosive having a very low detonation velocity, a high safety and an excellent sympathetic detonability without decreasing extremely its strength, and surprisingly found out that the use of a gas-retaining agent consisting of bubble assemblies, each bubble assembly being a secondary particle consisting of a large number of bubbles agglomerated into the particle, can produce a W/O explosive having a very low detonation velocity, and have reached the present invention.